Superconductor incorporating therein superconductivity epitaxial thin film and manufacturing method thereof

ABSTRACT

The present invention utilizes magnesium diboride (MgB 2 ) or (Mg 1−x M x )B 2  as a superconductivity thin film which can be applied to a rapid single flux quantum (RSFQ) circuit. A method for manufacturing a superconductor incorporating therein a superconductivity thin film, begins with preparing a single crystal substrate. Thereafter, a template film is formed on top of the substrate, wherein the template has a hexagonal crystal structure. The superconductivity thin film of MgB 2  or (Mg 1−x M x )B 2  is formed on top of the template film. If Mg amount in the superconductivity thin film is insufficient, Mg vapor is flowed on the surface of the superconductivity thin film while a post annealing process is carried out at the temperature ranging from 400° C. to 900° C.

FIELD OF THE INVENTION

The present invention relates to a superconductor; and, more particularly, to a superconductor incorporating therein an intermetallic compound superconductivity epitaxial thin film and a method for manufacturing the same, using a superconductor material such as magnesium diboride (MgB₂) or (Mg_(1−x)M_(x))B₂.

DESCRIPTION OF THE PRIOR ART

In general, a fabrication of a superconductivity thin film has been advanced for tens of years for the purpose of an electronic circuit application. Particularly, the fabrication of the thin film and its application to the electronic circuit has been mainly researched and developed by utilizing niobium (Nb) of a low temperature superconductor and Y₁Ba₂Cu₃O_(7−x) (YBCO) of a high temperature superconductor, wherein a superconductivity transition temperature (Tc) of Nb is 9.2 K. and that of the YBCO is 93 K.

The superconductivity transition temperature (Tc) of a YBCO thin film is higher than Tc, i.e., 77 K., of liquid nitrogen, and an energy gap of the YBCO thin film is greater than that of the low temperature superconductor so that it may be applied to the electronic circuit with a high speed performance. But, the YBCO thin film has a limitation that a uniform junction is too hard in a manufacturing process so that it is difficult to manufacture an integrated circuit (I.C).

On the contrary, Nb of the low temperature superconductor has advantages that the junction process is easy so that it may be applied to the fabrication of the I.C. However, an operation of the I.C is performed at a temperature below than the superconductivity transition temperature of liquid helium (He), i.e., about from 4 K. to 5 K., so that the Nb thin film is less practical in the view point of economy.

In recent years, magnesium diboride (MgB₂) is discovered as the superconductor material. The MgB₂ is an intermetallic compound superconductor having magnesium (Mg) and boron (B) therein. The composition of the MgB₂ is relatively simple and the superconductivity transition temperature is high, i.e., 39 K., so that it can be applied to the electronic circuit in case of fabricating MgB₂ as the thin film.

If MgB₂ is applied to the electronic circuit, the operation of the circuit can be performed at the temperature ranging from 15 K. to 20 K. using a conventional cryocooler and the speed of the circuit is approximately 4 times as fast as that of an Nb circuit which is operated at the temperature ranging from 4 K. to 5 K. In addition, since an operation temperature of the circuit ranges from 15 K. to 20 K., it is unnecessary to use liquid nitrogen so that it may be widely applied to an electronic device economically.

Up to now, several technologies for MgB₂ has been announced such as a fabrication technology of an MgB₂ powder, an MgB₂ pellet and an MgB₂ wire.

Referring to FIGS. 1 and 2 are graphs setting forth a X-ray diffraction pattern and a relation between the resistivity and the temperature of an MgB₂ thin film in accordance with a first prior art. According to the first prior art, the method for manufacturing the MgB₂ pellet begins with mixing Mg and B to a ratio of 1:2. Thereafter, mixture of Mg and B is pressurized at a high temperature in a hot isostatic pressing (HIP) furnace, thereby obtaining an MgB₂ pellet having the superconductivity transition temperature of 39 K. This is disclosed by J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani and J. Akimitsu in an article, “Superconductivity at 39 K. in Magnesium Diboride, Nature 410, 63, 2001”.

A second prior art for manufacturing the MgB₂ pellet is disclosed by C. U. Jung et al., in an article, “Temperature-and-Magnetic-Field-Dependences of Normal State Resistivity of MgB₂ Prepared at High Temperature and High Pressure Condition, http://www.lanl.gov/cond-mat/0102215”. In a disclosure, MgB₂ is fabricated in a type of pellet under the high temperature and the high pressure by using an anvil-typed press.

Furthermore, a third prior art for manufacturing the MgB₂ powder is disclosed by S. L. Bud'ko et al., in an article, “Boron Isotope Effect in Supercoducting MgB₂, Phys. Rev. Lett., 86, pp. 1,877-1,880, 2001”. According to the third prior art, to begin with, mixture of Mg and B is inserted into a tantalum (Ta) tube after Mg and B are mixed to a ratio of 1:2. Thereafter, the Ta tube is vacuum-sealed using a quartz capsule. Finally, the Ta tube provided with mixture of Mg and B therein is annealed at 950° C. and then cooled, thereby obtaining the MgB₂ powder.

A fourth prior art for manufacturing the MgB₂ wire is disclosed by P. C. Canfield et al., in an article, “Superconductivity in Dense MgB₂ wire, Phys. Rev. Lett., 86, pp. 2,423-2,426, 2001”. In a paper, boron fiber and Mg are inserted into the Ta tube and the tube is vacuum-sealed using the quartz capsule. Thereafter, it is annealed at 950° C., thereby obtaining the MgB₂ wire.

However, it is impossible for the MgB₂ powder and pellet and wire fabricated by the prior arts to be applied to the fabrication of the electronic circuit. Thus, a fabrication method for the MgB₂ thin film is required to be applied to the electronic circuit.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a superconductor incorporating therein a superconductivity epitaxial thin film of magnesium diboride (MgB₂) or (Mg_(1−x)M_(x))B₂ which can be applied to a rapid single flux quantum (RSFQ) circuit.

It is, therefore, another object of the present invention to provide a method for manufacturing a superconductor incorporating therein a superconductivity epitaxial thin film of MgB₂ or (Mg_(1−x)M_(x))B₂ which can be applied to a rapid single flux quantum (RSFQ) circuit.

In accordance with one aspect of the present invention, there is provided a superconductor comprising: a template film having a hexagonal crystal structure; and a superconductivity thin film formed on top of the template film, including magnesium (Mg) and boron (B) therein which are epitaxially grown up, wherein a crystal structure and a lattice constant of the template film are similar to those of the superconductivity thin film.

In accordance with another aspect of the present invention, there is provided a method for manufacturing a superconductor comprising the steps of: preparing a substrate; b) forming a template film on top of the substrate, wherein the template film has a hexagonal crystal structure; and c) forming a superconductivity thin film on top of the template film having Mg and B therein.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiment given in conjunction with the accompanying drawings, in which:

FIG. 1 is an X-ray diffraction pattern of magnesium diboride (MgB₂) pallet in accordance with a prior art;

FIG. 2 is a graph setting forth a relation between a resistivity and a temperature of the MgB₂ pallet in accordance with the prior art;

FIG. 3 is a schematic view setting forth an MgB₂ thin film or an (Mg_(1−x)M_(x))B₂ thin film in accordance with a first preferred embodiment of the present invention;

FIG. 4 is a schematic view setting forth the MgB₂ thin film or the (Mg_(1−x)M_(x))B₂ thin film in accordance with a second preferred embodiment of the present invention;

FIGS. 5A to 5C are schematic views illustrating an Mg_(1+x)B₂ target in accordance with the first and the second preferred embodiments of the present invention;

FIGS. 6A to 6C are schematic views of an (Mg_(1−x)M_(x))B₂ target in accordance with the first and the second preferred embodiments of the present invention; and

FIG. 7 is a phase diagram of MgB₂ in accordance with the first preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 3, there is shown a schematic view setting forth a superconductor incorporating therein a superconductivity epitaxial thin film having magnesium (Mg) and boron (B), in accordance with a first preferred embodiment of the present invention. The superconductor comprises a single crystal substrate 11 having a hexagonal crystal structure and the superconductivity epitaxial thin film 12 formed on top of the single crystal substrate 11.

Here, one of materials may be utilized as the single crystal substrate 11, as described in table 1.

TABLE 1 Material Lattice Constant (nm) Crystal Structure GaN 0.3180 hexagonal Al₂O₃ 0.24747 hexagonal SiC 0.3082 hexagonal ZnO 0.3250 hexagonal LiAlO₂ 0.3134 tetragonal LiGaO₂ 0.3186 orthorhombic

Referring to table 1, if the superconductivity epitaxial thin film 12 is magnesium diboride (MgB₂) of which the crystal structure is the hexagonal structure and lattice constant is 0.3086 nm, the material having the crystal structure and the lattice constant similar to those of MgB₂, is preferred to be used as the single crystal substrate 11, for example, gallium nitride (GaN), aluminum oxide (Al₂O₃), silicon carbide (SiC), zinc oxide (ZnO), LiAlO₂, LiGaO₂. Here, it is noted that LiAlO₂ and LiGaO₂ are a tetragonal and an orthorhombic structure respectively. But the lattice constants of LiAlO₂ and LiGaO₂ are similar to that of MgB₂ so that LiAlO₂ and LiGaO₂ may be used as the substrate for growing up MgB₂.

Referring to FIG. 4, there is shown a schematic view setting forth a superconductor incorporating therein a superconductivity epitaxial thin film having Mg and B, in accordance with a second preferred embodiment of the present invention. The superconductor comprises a substrate 21, a template film 22 formed on top of the substrate 21 and the superconductivity epitaxial thin film 23 formed on top of the template film 22. The substrate 21 includes a material such as silicon (Si), gallium arsenide (GaAs), metal, magnesium oxide (MgO) and strontium titanium oxide (SrTiO₃). The template film 22 uses LiAlO₂, LiGaO₂ or the material having the hexagonal crystal structure, wherein the template film 22 is used as a buffer layer or a seed layer. The material having the hexagonal structure is selected from the group including GaN, Al₂O₃, SiC and ZnO.

From the first and the second embodiments, it is understood that if material having the crystal structure and lattice constant similar to those of MgB₂, is used as the substrate or the template film, MgB₂ is epitaxially grown up with ease.

Instead of MgB₂ thin film as illustrated in the above embodiments, (Mg_(1−x)M_(x))B₂ thin film having the hexagonal crystal structure may also be used as the substrate or the template film, wherein M is a material selected from the group including copper (Cu), zinc (Zn), sodium (Na), beryllium (Be) and lithium (Li), and x denotes a rational number ranging from 0 to 1.

A method for manufacturing a superconductivity epitaxial thin film of MgB₂ or (Mg_(1−x)M_(x))B₂, is set forth in detail hereinafter.

To begin with, the MgB₂ thin film is formed by using a method such as a sputtering, a pulsed laser deposition, a chemical vapor deposition (CVD), a dual ion beam deposition, an E-beam evaporation or a spin coating technique. In case of using the pulsed laser deposition method, there is an advantage that the thin film having a composition similar to that of the target can be obtained because the superconductivity thin film is deposited after forming Ar-plasma on the surface of the substrate.

The CVD method has the advantage that it is appropriate for a mass production of a large size thin film because stoichiometry of Mg and B can be controlled by adjusting the flow of the carrier gas of Mg-organic material and B-organic material. By using the E-beam evaporation and the dual ion beam deposition, it is possible to obtain a high-grade thin film in a high vacuum state and further to control the stoichiometry of Mg and B independently.

Since the vapor pressure of Mg is high, a stoichiometric MgB₂ target or non-stoichiometric Mg_(1+x)B₂ target is used for forming the MgB₂ epitaxial thin film by using the sputtering, the pulsed laser deposition or the E-beam evaporation method. At this time, the MgB₂ target is a pellet-typed target which is made by pressurizing MgB₂ powder or pressurizing and heating MgB₂ powder at the same time. The Mg_(1+x)B₂ target is a sintered material in which Mg is added to MgB₂ powder, a mosaic-typed target using an Mg-metal plate and a B-metal plate, or an Mg-charged target in which Mg-metal plate is charged. In case of depositing by using the Mg_(1+x)B₂ target, the Mg_(1+x)B₂ target plays a role in supplementing insufficient amount of Mg. The Mg_(1+x)B₂ target is applied to the sputtering, the pulsed laser deposition or the E-beam evaporation method.

Referring to FIGS. 5A to 5C, there are shown schematic views illustrating the Mg_(1+x)B₂ target in accordance with the present invention, wherein FIG. 5A shows the sintered material in which Mg is added to MgB₂ powder, FIG. 5B is the mosaic-typed target using an Mg-metal plate and a B-metal plate, and FIG. 5C represents an Mg-charged target in which Mg-metal plate is charged.

Referring to 6A to 6C, there are shown schematic views of an (Mg_(1−x)M_(x))B₂ target in accordance with the present invention. The (Mg_(1−x)M_(x))B₂ target is selected from the group including a sintered (Mg_(1−x)M_(x))B₂ which the mixture of MgB₂ powder and M are pressurized and heated, an (Mg_(1−x)M_(x))B₂ mosaic-typed target using the Mg-metal plate, the B-metal plate and an M-metal plate, an (Mg_(1−x)M_(x))B₂-charged target which M is supplemented therein. Here, M denotes a material selected from the group including Cu, Zn, Na, Be and Li. The (Mg_(1−x)M_(x))B₂ target is used for the method such as the sputtering, the pulsed laser deposition or the E-beam evaporation technique.

Referring to FIG. 7, there is shown a phase diagram of MgB₂ in accordance with the present invention.

In order to fabricate the MgB₂ epitaxial thin film by using the above method, it is necessary to supply heat energy while the thin film is grown up. Therefore, the substrate is heated up to the temperature ranging from 400° C. to 900° C. where a L₁+MgB₂ phase is formed, as shown in FIG. 7.

Furthermore, it is preferable that the deposition of the thin film should be carried out in vacuum state, in argon (Ar) ambient atmosphere, in mixed gas of Ar and hydrogen (H₂) ambient atmosphere and in mixture of Ar and water vapor ambient atmosphere.

If the Mg amount of the thin film is insufficient after depositing the thin film, magnesium vapor is flowed on the surface of the thin film for supplementing Mg while a post annealing process is carried out at the temperature ranging from 400° C. to 900° C. The post annealing process should be carried out in the atmosphere without oxygen, such as in vacuum state, in argon (Ar) ambient atmosphere, in mixed gas of Ar and H₂ ambient atmosphere and in mixture of Ar and water vapor ambient atmosphere. Mg atoms supplemented during the post annealing process diffuse from the surface of the thin film into an Mg insufficient area, thereby achieving the stoichiometry of MgB₂.

Another method for manufacturing the MgB₂ epitaxial thin film begins with depositing a boron thin film on the substrate at a room temperature. Thereafter, Mg vapor is flowed on the surface of the boron thin film while the post annealing process is carried out at the temperature in the range of 400° C. to 900° C., thereby obtaining the MgB₂ thin film. Otherwise, an amorphous MgB₂ thin film is deposited on the substrate at the room temperature and then a passivation film is deposited on the amorphous MgB₂ thin film. Subsequently, the annealing process is carried out at the temperature ranging from 400° C. to 900° C., thereby obtaining the MgB₂ thin film.

In case of depositing the template film as the buffer layer or the seed layer, the deposition method is used such as the sputtering, the pulsed laser deposition, the CVD, the dual ion beam deposition, the E-beam evaporation or the spin coating technique.

In conclusion, in case of applying the MgB₂ or the (Mg_(1−x)M_(x))B₂ thin film of the present invention to a rapid single flux quantum (RSFQ) circuit, there is an advantage that the operation of the circuit can be performed at the temperature ranging from 15 K. to 20 K. using a conventional cryocooler. Furthermore, there is another advantage that the speed of the circuit is approximately 4 times as fast as that of an Nb circuit which is operated at the temperature ranging from 4 K to 5 K.

Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

What is claimed is:
 1. A method for manufacturing a superconductor comprising the steps of: a) preparing a substrate; b) forming a template film on top of the substrate, wherein the template film has a hexagonal crystal structure; and c) forming a superconductivity thin film on top of the template film having Mg and B therein.
 2. The method as recited in claim 1, wherein the step c) includes the steps of: c1) depositing a boron thin film on top of the template film; and c2) carrying out a post annealing process at a t=<ø{overscore ( )}{umlaut over ( )}Ó , øLkÈÁ+ temperature ranately 400°( C. to approximately) 900° C., while Mg vapor is being flowed on top of the boron thin film.
 3. The method as recited in claim 1, wherein the step c) includes the steps of: c1) depositing an amorphous thin film of MgB₂ on top of the template layer; c2) depositing a passivation film on top of the amorphous thin film of MgB₂ at a temperature below 400° C.; and c3) carrying out a post annealing process at the temperature ranging from approximately 400° C. to approximately 900° C.
 4. The method as recited in claim 3, wherein the passivation film includes a material selected from the group including MgO, GaN, SiC, ZnO and Al₂O₃.
 5. The method as recited in claim 1, wherein the substrate is made of a material selected from the group including Si, GaAs, metal, MgO, gallium nitride (GaN), aluminum oxide (Al₂O₃), silicon carbide (SiC), zinc oxide (Zno), LiAlO₂, LiGaO₂, magnesium oxide (MgO) and strontium titanium oxide (SrTiO₃).
 6. The method as recited in claim 1, wherein the superconductivity thin film is made of a material selected from the group including MgB₂ and (Mg_(1−x)M_(x))B₂, where, M is a material selected from the group including Cu, Zn, Na, Be and Li, and x is a rational number ranging from 0 to
 1. 7. The method as recited in claim 6, wherein the MgB₂ or the (Mg_(1−x)M_(x))B₂ is formed by using a method selected from the group including a sputtering, a pulsed laser deposition, a chemical vapor deposition, a dual ion beam deposition, an E-beam evaporation and a spin coating technique.
 8. The method as recited in claim 7, wherein the sputtering or the pulsed laser deposition is carried out in an atmosphere selected from the group including vacuum, argon (Ar) ambient, mixed gas of Ar and hydrogen (H₂) and mixture of Ar and water vapor.
 9. The method as recited in claim 7, wherein a target used for the sputtering, the pulsed laser deposition or the E-beam evaporation technique is a stoichiometric MgB₂ target or a non-stoichiometric Mg_(1+x)B₂ target, where, x is a rational number that is not a negative number.
 10. The method as recited in claim 9, wherein the MgB₂ target is a pellet typed target which MgB₂ powder is pressurized or a pellet typed target which is heated and pressurized simultaneously.
 11. The method as recited in claim 9, wherein the Mg_(1+x)B₂ target is selected from the group including a sintered material which Mg is added with MgB₂ powder, a mosaic-typed target using an Mg-metal plate and a B-metal plate, and an Mg-charged target in which Mg-metal plate is charged.
 12. The method as recited in claim 7, wherein the (Mg_(1−x)M_(x))B₂ target is selected from the group including a sintered (Mg_(1−x)M_(x))B₂, a (Mg_(1−x)M_(x))B₂ mosaic-typed target and a (Mg_(1−x)M_(x))B₂-charged target.
 13. The method as recited in claim 1, after the step c), further comprising the step of carrying out a post annealing process at the temperature ranging from approximately 400° C. to approximately 900° C. while Mg vapor is flowed on the surface of the superconductivity thin film providing that Mg amount in the superconductivity thin film is insufficient.
 14. The method as recited in claim 13, the annealing process is carried out in the atmosphere selected from the group including vacuum, Ar ambient, mixed gas of Ar and H₂ and mixture of Ar and water vapor.
 15. The method as recited in claim 1, wherein the step b) is carried out while the substrate is being heated up to the temperature ranging from approximately 400° C. to approximately 900° C. 